In internal combustion engines, such as automotive engines, a spark plug is equipped with a center electrode and a ground electrode which are commonly made of Ni alloy, etc. Ni alloy has slightly less oxidation resistance and erosion resistance than those of a noble metal alloy containing noble metals such as Pt and Ir as a main component. However, the cost of Ni alloy is lower than that of noble metals, and therefore, is preferably used as a material for the ground and center electrodes.
The internal temperature of a combustion chamber has in recent years tended to be increased. Therefore, when spark discharge occurs between front end portions of the ground and center electrodes which are made of Ni alloy, etc., the erosion of the front end portions, facing each other, of the ground and center electrodes is likely to occur due to sparking. Therefore, in order to improve the erosion resistance of the ground and center electrodes, a technique has been developed in which a tip is provided on each of the front end portions, facing each other, of the ground and center electrodes, so that spark discharge occurs on these tips.
The tip is typically made of a material containing, as a main component, a noble metal having excellent oxidation resistance and spark erosion resistance. Examples of such a material include Ir, Ir alloy, and Pt alloy. A tip containing Ir as a main component is known to have excellent spark erosion resistance, and is also known to undergo abnormal erosion such that an outer peripheral surface of the tip which is not a discharge surface is hollowed into the shape of an arc. In order to inhibit such abnormal erosion, a tip has been proposed in which a coating layer containing Ni is provided on the surface of the body portion containing Ir as a main component (e.g., Japanese Patent Application Laid-Open (kokai) No. 2004-127681 and Japanese Patent Application Laid-Open (kokai) No. 2004-31300).
Incidentally, as to spark plugs, the internal temperature of a combustion chamber has in recent years tended to be increased in order to further improve the power output or fuel efficiency of an engine. When a technology such as a start-stop system is employed, the number of times an engine is turned on or off increases, and therefore, the number of heating/cooling cycles increases. In addition, the internal temperature range of the combustion chamber increases, for example. Thus, a spark plug is exposed to a harsher heating/cooling cycle environment. Furthermore, in order to improve the ignition performance or dielectric strength of an engine, a spark plug has been employed which is equipped with tips narrower than the ground and center electrodes. Therefore, compared to conventional spark plugs, the heating/cooling cycle environment is harsh for such a spark plug even when the traditional cycle from full throttle to idling is only repeatedly performed, assuming that the engine environment is the same. Therefore, the spark plug has begun to need durability at high temperature.
It has been found that when a spark plug for use in a combination of such a high temperature environment and harsh heating/cooling cycle environment is used for a long time so that the maintenance interval is elongated, then even if the spark plug is one equipped with a tip which is disclosed in Japanese Patent Application Laid-Open (kokai) No. 2004-127681 or Japanese Patent Application Laid-Open (kokai) No. 2004-31300, the abnormal erosion of the spark plug is unlikely to be inhibited. In other words, it has been found that when a spark plug is used in the above harsh environment, the abnormal-erosion resistance of the tip is likely to decrease after a predetermined period of time has elapsed.
An advantage of the present invention is a spark plug having excellent durability which is characterized in that a tip is provided on at least one of the center and ground electrodes of the spark plug, and when the spark plug is used in a combination of a high-temperature environment and a harsh heating/cooling cycle environment, the abnormal erosion of the tip is inhibited for a long time.